The present invention is directed to countermeasures devices and in particular to an improved passive radar decoy capable of projecting a uniformly large radar cross section over a wide angle of incidence.
Since the inception of radar guided weapon systems as viable threats to airborne platforms, much effort has been concentrated on developing countermeasures against such threats. The use of chaff and electronic jamming have been developed and employed by airborne platforms as effective countermeasures against the radar assisted weapon systems.
Ground and sea based installations are also susceptible to attack from radar guided weapons and passive countermeasure devices are being developed to provide protection from the radar assisted threats. The purpose of the passive countermeasure devices is to either conceal a potential target from detection by a surveillance type radar or to serve as a decoy against radar guided missiles. The concealment type devices are designed to enhance the radar clutter environment surrounding a target and should produce radar returns that are indistinguishable from the target it is protecting. The decoy type countermeasure device is designed to shift or relocate the radar centroid of the target as seen by the radar guided missile thereby causing the missile to miss its intended target.
A characteristic of all passive radar decoys is that when energy from a radar guided weapon impinges on their surface they must be capable of reradiating a large amount of this energy in the direction of the radar's receiver. A measure of the amount of radar energy incident on a target that is reradiated in the direction of the radar's receive antenna is known as the target's radar cross section (RCS). If the radar's transmit and receive antennas are located together the RCS is called monostatic RCS, otherwise the term bistatic RCS is used. Typically, the radar decoy is designed to present RCS that is much greater than the object it is designed to protect.
A second characteristic of a passive radar decoy is that the large RCS remains relatively large over a wide range of radar operating frequencies. Additionally, if the radar decoy is to function without prior knowledge as to the location of the radar threat, i.e. no decoy steering capability is employed, it must be capable of providing the large RCS characteristic over a wide range of radar viewing angles.
The basic building blocks used in the construction of typical radar decoys are the three-sided (trihedral) triangular corner reflectors. The corner retro-reflectors are most frequently used because of their ability to redirect a large portion of the incident radar energy back toward a monostatic radar and over a wide range of radar viewing angles. Traditionally, the individual corner reflectors have been arranged into a set of eight corner reflectors by effectively placing the reflectors back-to-back with four reflectors on top and four inverted reflectors on the bottom.
An example of an eight corner reflector is shown in U.S. Pat. No. 4,119,965 which discloses a foldable radar target such as might be utilized on small boats. The radar target comprises a base plate to which four top quarter plates and four bottom quarter plates are hingedly connected along first base sides thereof. The quarter panels are movable from a face-to-face collapsed position with respect to the base plate to an upright attitude, in which second base sides of each top and bottom quarter panel are respectively aligned proximal to each other along a line that is generally vertical and perpendicular to the normal horizontal disposition of the base plate.
Other examples of eight corner back-to-back retro-reflectors are shown in U.S. Pat. Nos. 3,103,662 and 2,450,417.
A significant deficiency of the prior art eight corner reflectors, lies in the fact that any one of the individual retro-reflectors operates as a reflector only over a solid angle of about 60.degree., so that when one installs them into a back-to-back configuration where they physically occupy 90.degree. and work only over 60.degree., a large part of the desired spherical coverage is either seriously degraded or non-existent. The 60.degree. limit on the functional properties of a single retro-reflector can best be visualized by looking into a corner and observing the three orthogonal sides that comprise the corner. Realizing that retro-reflection occurs only when electromagnetic rays strike all three surfaces, one can slowly rotate the corner in any direction and notice the reduction in the visible portion of at least one of the three surfaces. This visible reduction is a direct measure of the loss of performance of the retro-reflector. When rotated to the point where one side disappears totally, the unit has been reduced to the functional characteristics of a two-sided corner reflector. At only exactly this angle will the two-sided corner reflector provide an excellent reflective capability and at all other rotational angles it will not. Such deficiencies in total spherical coverage cannot be tolerated in defense applications where battle strategy and survivability of personnel and equipment depend on full, total, angular coverage by the decoy.